diff --git a/aiogram/client/session/aiohttp.py b/aiogram/client/session/aiohttp.py
index 9e1d9e1b..69f36bf1 100644
--- a/aiogram/client/session/aiohttp.py
+++ b/aiogram/client/session/aiohttp.py
@@ -23,10 +23,11 @@ from aiohttp.http import SERVER_SOFTWARE
 
 from aiogram.__meta__ import __version__
 from aiogram.methods import TelegramMethod
-from .base import BaseSession
+
 from ...exceptions import TelegramNetworkError
 from ...methods.base import TelegramType
 from ...types import InputFile
+from .base import BaseSession
 
 if TYPE_CHECKING:
     from ..bot import Bot
diff --git a/aiogram/types/custom.py b/aiogram/types/custom.py
index 70a62ded..094fd68c 100644
--- a/aiogram/types/custom.py
+++ b/aiogram/types/custom.py
@@ -1,13 +1,27 @@
+import sys
 from datetime import datetime
 
 from pydantic import PlainSerializer
 from typing_extensions import Annotated
 
+if sys.platform == "win32":  # pragma: no cover
+
+    def _datetime_serializer(value: datetime) -> int:
+        # https://github.com/aiogram/aiogram/issues/349
+        # https://github.com/aiogram/aiogram/pull/880
+        return round((value - datetime(1970, 1, 1)).total_seconds())
+
+else:  # pragma: no cover
+
+    def _datetime_serializer(value: datetime) -> int:
+        return round(value.timestamp())
+
+
 # Make datetime compatible with Telegram Bot API (unixtime)
 DateTime = Annotated[
     datetime,
     PlainSerializer(
-        func=lambda dt: int(dt.timestamp()),
+        func=_datetime_serializer,
         return_type=int,
         when_used="unless-none",
     ),